Catalytic depolymerization of polymers containing electrophilic linkages using nucleophilic reagents

ABSTRACT

A method is provided for carrying out depolymerization of a polymer containing electrophilic linkages in the presence of a catalyst and a nucleophilic reagent, wherein production of undesirable byproducts resulting from polymer degradation is minimized. The reaction can be carried out at a temperature of 80° C. or less, and generally involves the use of an organic, nonmetallic catalyst, thereby ensuring that the depolymerization product(s) are substantially free of metal contaminants. In an exemplary depolymerization method, the catalyst is a carbene compound such as an N-heterocyclic carbene, or is a precursor to a carbene compound. The method provides an important alternative to current recycling techniques such as those used in the degradation of polyesters, polyamides, and the like.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This is a divisional of U.S. patent application Ser. No.10/330,853 filed Dec. 26, 2002, the disclosure of which is incorporatedby reference in its entirety.

ACKNOWLEDGEMENT OF GOVERNMENT SUPPORT

[0002] This invention was made in part with Government support under agrant from the National Science Foundation (Cooperative Agreement No.DMR-980677). Accordingly, the Government may have certain rights to thisinvention.

TECHNICAL FIELD

[0003] This invention relates generally to the depolymerization ofpolymers, and, more particularly relates to an organocatalytic methodfor depolymerizing polymers using nucleophilic reagents. The inventionis applicable in numerous fields, including industrial chemistry andchemical waste disposal, plastics recycling, and manufacturing processesrequiring a simple and convenient method for the degradation ofpolymers.

BACKGROUND OF THE INVENTION

[0004] Technological advances of all kinds continue to present manycomplex ecological issues. Consequently, waste management and pollutionprevention are two very significant challenges of the 21^(st) century.The overwhelming quantity of plastic refuse has significantlycontributed to the critical shortage of landfill space faced by manycommunities. For example, poly(ethylene terephthalate)(poly(oxy-1,2-ethanediyl-oxycarbonyl-1,4-diphenylenecarbonyl); “PET”), awidely used engineering thermoplastic for carpeting, clothing, tirecords, soda bottles and other containers, film, automotive applications,electronics, displays, etc. will contribute more than 1 billion poundsof waste to land-fills in 2002 alone. The worldwide production of PEThas been growing at an annual rate of 10% per year, and with theincrease in use in electronic and automotive applications, this rate isexpected to increase significantly to 15% per year. Interestingly, theprecursor monomers represent only about 2% of the petrochemical stream.Moreover, the proliferation of the use of organic solvents, halogenatedsolvents, water, and energy consumption in addressing the need torecycle commodity polymers such as PET and other polyesters has createdthe need for environmentally responsible and energy efficient recyclingprocesses. See Nadkarni (1999) International Fiber Journal 14(3).

[0005] Significant effort has been invested in researching recyclingstrategies for PET, and these efforts have produced three commercialoptions; mechanical, chemical and energy recycling. Energy recyclingsimply burns the plastic for its calorific content. Mechanicalrecycling, the most widespread approach, involves grinding the polymerto powder, which is then mixed with “virgin” PET. See Mancini et al.(1999) Materials Research 2(1):33-38. Many chemical companies use thisprocess in order to recycle PET at the rate of approximately 50,000tons/year per plant. In Europe, all new packaging materials as of 2002must contain 15% recycled material. However, it has been demonstratedthat successive recycling steps cause significant polymer degradation,in turn resulting in a loss of desirable mechanical properties.Recycling using chemical degradation involves a process thatdepolymerizes a polymer to starting material, or at least to relativeshort oligomeric components. Clearly, this process is most desirable,but is the most difficult to control since elevated temperature andpressure are required along with a catalyst composed of a strong base,or an organometallic complex such as an organic titanate. See Sako etal. (1997) Proc. of the 4^(th) Int'l Symposium on Supercritical Fluids,pp. 107-110. The use of such a catalyst results in significantquantities of undesirable byproducts, and materials processed by thesemethods are thus generally unsuitable for use in medical materials orfood packaging, limiting their utility. Moreover, the energy required toeffect depolymerization essentially eliminates sustainability arguments.

[0006] Accordingly, there is a need in the art for an improveddepolymerization method. Ideally, such a method would not involveextreme reaction conditions, use of metallic catalysts, or a processthat results in significant quantities of potentially problematicby-products.

SUMMARY OF THE INVENTION

[0007] The invention is directed to the aforementioned need in the art,and, as such, provides an efficient catalytic depolymerization reactionthat employs mild conditions, wherein production of undesirablebyproducts resulting from polymer degradation is minimized. The reactioncan be carried out at temperatures of at most 80° C., and, because anonmetallic catalyst is preferably employed, the depolymerizationproducts, in a preferred embodiment, are substantially free of metalcontaminants. With many of the carbene catalysts disclosed herein, thedepolymerization reaction can be carried out at a temperature of at most60° C. or even 30° C. or lower, i.e., at room temperature.

[0008] More specifically, in one aspect of the invention, a method isprovided for depolymerizing a polymer having a backbone containingelectrophilic linkages, wherein the method involves contacting thepolymer with a nucleophilic reagent and a catalyst at a temperature ofless than 80° C. An important application of this method is in thedepolymerization of polyesters, including homopolymeric polyesters (inwhich all of the electrophilic linkages are ester linkages) andpolyester copolymers (in which a fraction of the electrophilic linkagesare ester linkages and the remainder of the electrophilic linkages areother than ester linkages).

[0009] In a related aspect of the invention, a method is provided fordepolymerizing a polymer having a backbone containing electrophiliclinkages, wherein the method involves contacting the polymer with anucleophilic reagent and a catalyst that yields depolymerizationproducts that are substantially free of metal contaminants. The polymermay be, for example, a polyester, a polycarbonate, a polyurethane, or arelated polymer, in either homopolymeric or copolymeric form, asindicated above. In this embodiment, in order to provide reactionproducts that are substantially free of contamination by metals andmetal-containing compounds, the catalyst used is a purely organic,nonmetallic catalyst. Preferred catalysts herein are carbene compounds,which act as nucleophilic catalysts, as well as precursors to carbenecompounds, as will be discussed infra. As is well understood in the art,carbenes are electronically neutral compounds containing a divalentcarbon atom with only six electrons in its valence shell. Carbenesinclude, by way of example, cyclic diaminocarbenes, imidazol-2-ylidenes(e.g., 1,3-dimesityl-imidazol-2-ylidene and1,3-dimesityl-4,5-dihydroimidazol-2-ylidene), 1,2,4-triazol-3-ylidenes,and 1,3-thiazol-2-ylidenes; see Bourissou et al. (2000) Chem. Rev.100:39-91.

[0010] Since the initial description of the synthesis, isolation, andcharacterization of stable carbenes by Arduengo (Arduengo et al. (1991)J. Am. Chem. Soc. 113:361; Arduengo et al. (1992) J. Am. Chem. Soc.114:5530), the exploration of their chemical reactivity has become amajor area of research. See, e.g., Arduengo et al. (1999) Acc. Chem.Res. 32:913; Bourissou et al. (2000), supra; and Brode (1995) Angew.Chem. Int. Ed. Eng. 34:1021. Although carbenes have now been extensivelyinvestigated, and have in fact been established as useful in manysynthetically important reactions, there has been no disclosure orsuggestion to use carbenes as catalysts in nucleophilic depolymerizationreactions, i.e., reactions in which a polymer containing electrophiliclinkages is depolymerized with a nucleophilic reagent in the presence ofa carbene catalyst.

[0011] Suitable catalysts for use herein thus includeheteroatom-stabilized carbenes or precursors to such carbenes. Theheteroatom-stabilized carbenes have the structure of formula (I)

[0012] wherein:

[0013] E¹ and E² are independently selected from N, NR^(E), O, P,PR^(E), and S, R^(E) is hydrogen, heteroalkyl, or heteroaryl, x and yare independently zero, 1, or 2, and are selected to correspond to thevalence state of E¹ and E², respectively, and wherein when E¹ and E² areother than O or S, then E¹ and E² may be linked through a linking moietythat provides a heterocyclic ring in which E¹ and E² are incorporated asheteroatoms;

[0014] R¹ and R² are independently selected from branched C₃-C₃₀hydrocarbyl, substituted branched C₃-C₃₀ hydrocarbyl,heteroatom-containing branched C₄-C₃₀ hydrocarbyl, substitutedheteroatom-containing branched C₄-C₃₀ hydrocarbyl, cyclic C₅-C₃₀hydrocarbyl, substituted cyclic C₅-C₃₀ hydrocarbyl,heteroatom-containing cyclic C₁-C₃₀ hydrocarbyl, and substitutedheteroatom-containing cyclic C₁-C₃₀ hydrocarbyl;

[0015] L¹ and L² are linkers containing 1 to 6 spacer atoms, and areindependently selected from heteroatoms, substituted heteroatoms,hydrocarbylene, substituted hydrocarbylene, heteroatom-containinghydrocarbylene, and substituted heteroatom-contraining hydrocarbylene;and

[0016] m and n are independently zero or 1, such that L¹ and L² areoptional.

[0017] Certain carbene catalysts of formula (1) are novel chemicalcompounds and are claimed as such herein. These novel carbenes are thosewherein a heteroatom is directly bound to E¹ and/or E², and include,solely by way of example, carbenes of formula (I) wherein E¹ is NR^(E)and R^(E) is a heteroalkyl or heteroaryl group such as an alkoxy,alkylthio, aryloxy, arylthio, aralkoxy, or aralkylthio moiety.

[0018] Carbene precursors suitable as catalysts herein includetri-substituted methanes having the structure of formula (PI), metaladducts having the structure of formula (PII), and tetrasubstitutedolefins having the structure (PIII)

[0019] wherein, in formulae (PI) and (PII):

[0020] E¹ and E² are independently selected from N, NR^(E), O, P,PR^(E), and S, R^(E) is hydrogen, heteroalkyl, or heteroaryl, x and yare independently zero, 1, or 2, and are selected to correspond to thevalence state of E¹ and E², respectively, and wherein when E¹ and E² areother than O or S, then E¹ and E² may be linked through a linking moietythat provides a heterocyclic ring in which E¹ and E² are incorporated asheteroatoms;

[0021] R¹ and R² are independently selected from branched C₃-C₃₀hydrocarbyl, substituted branched C₃-C₃₀ hydrocarbyl,heteroatom-containing branched C₄-C₃₀ hydrocarbyl, substitutedheteroatom-containing branched C₄-C₃₀ hydrocarbyl, cyclic C₅-C₃₀hydrocarbyl, substituted cyclic C₅-C₃₀ hydrocarbyl,heteroatom-containing cyclic C₁-C₃₀ hydrocarbyl, and substitutedheteroatom-containing cyclic C₁-C₃₀ hydrocarbyl;

[0022] L¹ and L² are linkers containing 1 to 6 spacer atoms, and areindependently selected from heteroatoms, substituted heteroatoms,hydrocarbylene, substituted hydrocarbylene, heteroatom-containinghydrocarbylene, and substituted heteroatom containing hydrocarbylene;

[0023] m and n are independently zero or 1;

[0024] R⁷ is selected from alkyl, heteroalkyl, aryl, heteroaryl,aralkyl, or heteroaralkyl, substituted with at least oneelectron-withdrawing substituent;

[0025] M is a metal;

[0026] Ln is a ligand; and

[0027] j is the number of ligands bound to M.

[0028] In compounds of formula (PIII), the substituents are as follows:

[0029] E³ and E⁴ are defined as for E¹ and E²;

[0030] v and w are defined as for x and y;

[0031] R⁸ and R⁹ are defined as for R¹ and R²;

[0032] L³ and L⁴ are defined as for L¹ and L²; and

[0033] h and k are defined as for m and n.

[0034] The carbene precursors may be in the form of a salt, in whichcase the precursor is positively charged and associated with an anioniccounterion, such as a halide ion (I, Br, Cl), a hexafluorophosphateanion, or the like.

[0035] Novel carbene precursors herein include compounds of formula(PII), those compounds of formula (PIII) in which a heteroatom isdirectly bound to E¹ and/or E², and those compounds of formula (PII) inwhich a heteroatom is directly bound to at least one of E¹, E², E³, andE⁴, and may be in the form of a salt as noted above.

[0036] Ideally, the carbene catalyst used in conjunction with thepresent depolymerization reaction is an N-heterocyclic carbene havingthe structure of formula (II)

[0037] wherein:

[0038] R¹, R², L¹, L², m, and n are as defined above; and

[0039] L is a hydrocarbylene, substituted hydrocarbylene,heteroatom-containing hydrocarbylene, or substitutedheteroatom-containing hydrocarbylene linker, wherein two or moresubstituents on adjacent atoms within L may be linked to form anadditional cyclic group.

[0040] As alluded to above, one important application of the presentinvention is in the recycling of polyesters, including, by way ofillustration and not limitation: PET; poly (butylene terephthalate)(PBT); poly(alkylene adipate)s and their copolymers; andpoly(ε-caprolactone). The methodology of the invention provides anefficient means to degrade such polymers into their component monomersand/or relatively short oligomeric fragments without need for extremereaction conditions or metallic catalysts.

BRIEF DESCRIPTION OF THE DRAWINGS

[0041]FIG. 1 illustrates the organocatalytic depolymerization of PET inthe presence of excess methanol using N-heterocyclic carbene catalyst,as evaluated in Example 7.

[0042]FIG. 2 illustrates the organocatalytic depolymerization of PET inthe presence of ethylene glycol using N-heterocyclic carbene catalyst,as evaluated in Example 6.

DETAILED DESCRIPTION OF THE INVENTION

[0043] Unless otherwise indicated, this invention is not limited tospecific polymers, carbene catalysts, nucleophilic reagents, ordepolymerization conditions. The terminology used herein is for thepurpose of describing particular embodiments only and is not intended tobe limiting.

[0044] As used in the specification and the appended claims, thesingular forms “a,” “an,” and “the” include plural referents unless thecontext clearly dictates otherwise. Thus, for example, reference to “apolymer” encompasses a combination or mixture of different polymers aswell as a single polymer, reference to “a catalyst” encompasses both asingle catalyst as well as two or more catalysts used in combination,and the like.

[0045] In this specification and in the claims that follow, referencewill be made to a number of terms, which shall be defined to have thefollowing meanings:

[0046] As used herein, the phrase “having the formula” or “having thestructure” is not intended to be limiting and is used in the same waythat the term “comprising” is commonly used.

[0047] The term “alkyl” as used herein refers to a linear, branched, orcyclic saturated hydrocarbon group typically although not necessarilycontaining 1 to about 20 carbon atoms, preferably 1 to about 12 carbonatoms, such as methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl,t-butyl, octyl, decyl, and the like, as well as cycloalkyl groups suchas cyclopentyl, cyclohexyl and the like. Generally, although again notnecessarily, alkyl groups herein contain 1 to about 12 carbon atoms. Theterm “lower alkyl” intends an alkyl group of 1 to 6 carbon atoms, andthe specific term “cycloalkyl” intends a cyclic alkyl group, typicallyhaving 4 to 8, preferably 5 to 7, carbon atoms. The term “substitutedalkyl” refers to alkyl substituted with one or more substituent groups,and the terms “heteroatom-containing alkyl” and “heteroalkyl” refer toalkyl in which at least one carbon atom is replaced with a heteroatom.If not otherwise indicated, the terms “alkyl” and “lower alkyl” includelinear, branched, cyclic, unsubstituted, substituted, and/orheteroatom-containing alkyl and lower alkyl, respectively.

[0048] The term “alkylene” as used herein refers to a difunctionallinear, branched, or cyclic alkyl group, where “alkyl” is as definedabove.

[0049] The term “alkenyl” as used herein refers to a linear, branched,or cyclic hydrocarbon group of 2 to about 20 carbon atoms containing atleast one double bond, such as ethenyl, n-propenyl, isopropenyl,n-butenyl, isobutenyl, octenyl, decenyl, tetradecenyl, hexadecenyl,eicosenyl, tetracosenyl, and the like. Preferred alkenyl groups hereincontain 2 to about 12 carbon atoms. The term “lower alkenyl” intends analkenyl group of 2 to 6 carbon atoms, and the specific term“cycloalkenyl” intends a cyclic alkenyl group, preferably having 5 to 8carbon atoms. The term “substituted alkenyl” refers to alkenylsubstituted with one or more substituent groups, and the terms“heteroatom-containing alkenyl” and “heteroalkenyl” refer to alkenyl inwhich at least one carbon atom is replaced with a heteroatom. If nototherwise indicated, the terms “alkenyl” and “lower alkenyl” includelinear, branched, cyclic, unsubstituted, substituted, and/orheteroatom-containing alkenyl and lower alkenyl, respectively.

[0050] The term “alkenylene” as used herein refers to a difunctionallinear, branched, or cyclic alkenyl group, where “alkenyl” is as definedabove.

[0051] The term “alkoxy” as used herein refers to a group —O-alkylwherein “alkyl” is as defined above, and the term “alkylthio” as usedherein refers to a group —S-alkyl wherein “alkyl is as defined above.

[0052] The term “aryl” as used herein, and unless otherwise specified,refers to an aromatic substituent containing a single aromatic ring ormultiple aromatic rings that are fused together, directly linked, orindirectly linked (such that the different aromatic rings are bound to acommon group such as a methylene or ethylene moiety). Preferred arylgroups contain 5 to 20 carbon atoms and either one aromatic ring or 2 to4 fused or linked aromatic rings, e.g., phenyl, naphthyl, biphenyl, andthe like, with more preferred aryl groups containing 1 to 3 aromaticrings, and particularly preferred aryl groups containing 1 or 2 aromaticrings and 5 to 14 carbon atoms. “Substituted aryl” refers to an arylmoiety substituted with one or more substituent groups, and the terms“heteroatom-containing aryl” and “heteroaryl” refer to aryl in which atleast one carbon atom is replaced with a heteroatom. Unless otherwiseindicated, the terms “aromatic,” “aryl,” and “arylene” includeheteroaromatic, substituted aromatic, and substituted heteroaromaticspecies.

[0053] The term “aryloxy” refers to a group —O-aryl wherein “aryl” is asdefined above.

[0054] The term “alkaryl” refers to an aryl group with at least one andtypically 1 to 6 alkyl, preferably 1 to 3, alkyl substituents, and theterm “aralkyl” refers to an alkyl group with an aryl substituent,wherein “aryl” and “alkyl” are as defined above. Alkaryl groups include,for example, p-methylphenyl, 2,4-dimethylphenyl, 2,4,6-trimethylphenyl,and the like. The term “aralkyl” refers to an alkyl group substitutedwith an aryl moiety, wherein “alkyl” and “aryl” are as defined above.

[0055] The term “alkaryloxy” refers to a group —O—R wherein R isalkaryl, the term “alkarylthio” refers to a group —S—R wherein R isalkaryl, the term aralkoxy refers to a group —O—R wherein R is aralkyl,the term “aralkylthio” refers to a group —S—R wherein R is aralkyl.

[0056] The terms “halo,” “halide,” and “halogen” are used in theconventional sense to refer to a chloro, bromo, fluoro, or iodosubstituent. The terms “haloalkyl,” “haloalkenyl,” and “haloalkynyl” (or“halogenated alkyl,” “halogenated alkenyl,” and “halogenated alkynyl”)refer to an alkyl, alkenyl, or alkynyl group, respectively, in which atleast one of the hydrogen atoms in the group has been replaced with ahalogen atom.

[0057] “Hydrocarbyl” refers to univalent hydrocarbyl radicals containing1 to about 30 carbon atoms, preferably 1 to about 20 carbon atoms, morepreferably 1 to about 12 carbon atoms, including linear, branched,cyclic, saturated, and unsaturated species, such as alkyl groups,alkenyl groups, aryl groups, alkaryl groups, and the like. The term“lower hydrocarbyl” intends a hydrocarbyl group of 1 to 6 carbon atoms,and the term “hydrocarbylene” intends a divalent hydrocarbyl moietycontaining 1 to about 30 carbon atoms, preferably 1 to about 20 carbonatoms, most preferably 1 to about 12 carbon atoms, including linear,branched, cyclic, saturated and unsaturated species. The term “lowerhydrocarbylene” intends a hydrocarbylene group of 1 to 6 carbon atoms.Unless otherwise indicated, the terms “hydrocarbyl” and “hydrocarbylene”are to be interpreted as including substituted and/orheteroatom-containing hydrocarbyl and hydrocarbylene moieties,respectively.

[0058] The term “heteroatom-containing” as in a “heteroatom-containingalkyl group” (also termed a “heteroalkyl” group) or a“heteroatom-containing aryl group” (also termed a “heteroaryl” group)refers to a molecule, linkage, or substituent in which one or morecarbon atoms are replaced with an atom other than carbon, e.g.,nitrogen, oxygen, sulfur, phosphorus or silicon, typically nitrogen,oxygen or sulfur. Similarly, the term “heteroalkyl” refers to an alkylsubstituent that is heteroatom-containing, the term “heterocyclic”refers to a cyclic substituent that is heteroatom-containing, the terms“heteroaryl” and heteroaromatic” respectively refer to “aryl” and“aromatic” substituents that are heteroatom-containing, and the like.Examples of heteroalkyl groups include alkoxyaryl,alkylsulfanyl-substituted alkyl, N-alkylated amino alkyl, and the like.Examples of heteroaryl substituents include pyrrolyl, pyrrolidinyl,pyridinyl, quinolinyl, indolyl, pyrimidinyl, imidazolyl,1,2,4-triazolyl, tetrazolyl, etc., and examples of heteroatom-containingalicyclic groups are pyrrolidino, morpholino, piperazino, piperidino,etc. It should be noted that a “heterocyclic” group or compound may ormay not be aromatic, and further that “heterocycles” may be monocyclic,bicyclic, or polycyclic as described above with respect to the term“aryl.”

[0059] By “substituted” as in “substituted hydrocarbyl,” “substitutedalkyl,” “substituted aryl,” and the like, as alluded to in some of theaforementioned definitions, is meant that in the hydrocarbyl, alkyl,aryl, or other moiety, at least one hydrogen atom bound to a carbon (orother) atom is replaced with a non-hydrogen substituent. Examples ofsuch substituents include, without limitation, functional groups such ashalide, hydroxyl, sulfhydryl, C₁-C₂₀ alkoxy, C₅-C₂₀ aryloxy, C₂-C₂₀ acyl(including C₂-C₂₀ alkylcarbonyl (—CO-alkyl) and C₆-C₂₀ arylcarbonyl(—CO-aryl)), acyloxy (—O-acyl), C₂-C₂₀ alkoxycarbonyl (—(CO)—O-alkyl),C₆-C₂₀ aryloxycarbonyl (—(CO)—O-aryl), halocarbonyl (—CO)—X where X ishalo), C₂-C₂₀ alkyl-carbonato (—O—(CO)—O-alkyl), C₆-C₂₀ arylcarbonato(—O—(CO)—O-aryl), carboxy (—COOH), carboxylato (—COO⁻), carbamoyl(—(CO)—NH₂), mono-(C₁-C₂₀ alkyl)-substituted carbamoyl (—(CO)—NH(C₁-C₂₀alkyl)), di-(C₁-C₂₀ alkyl)-substituted carbamoyl —(CO)—N(C₁-C₂₀alkyl)₂), mono-substituted arylcarbamoyl (—(CO)—NH-aryl), thiocarbamoyl(—(CS)—NH₂), carbamido (—NH—(CO)—NH₂), cyano(—C═N), cyanato (—O—C≡N),formyl (—(CO)—H), thioformyl (—(CS)—H), amino (—NH²), mono- anddi-(C₁-C₂₀ alkyl)-substituted amino, mono- and di-(C₅-C₂₀aryl)-substituted amino, C₂-C₂₀ alkylamido (—NH—(CO)-alkyl), C₆-C₂₀arylamido (—NH—(CO)-aryl), imino (—CR═NH where R=hydrogen, C₁-C₂₀ alkyl,C₅-C₂₀ aryl, C₆-C₂₄ alkaryl, C₆-C₂₄ aralkyl, etc.), alkylimino(—CR═N(alkyl), where R=hydrogen, alkyl, aryl, alkaryl, etc.), arylimino(—CR═N(aryl), where R=hydrogen, alkyl, aryl, alkaryl, etc.), nitro(—NO₂), nitroso (—NO), sulfo (—SO₂—OH), sulfonato (—SO₂—O—), C₁-C₂₀alkylsulfanyl (—S-alkyl; also termed “alkylthio”), arylsulfanyl(—S-aryl; also termed “arylthio”), C₁-C₂₀ alkylsulfinyl (SO)-alkyl),C₅-C₂₀ arylsulfinyl (—(SO)-aryl), C₁-C₂₀ alkylsulfonyl (—SO₂-alkyl),C₅-C₂₀ arylsulfonyl (—SO₂-aryl), and thiocarbonyl (═S); and thehydrocarbyl moieties C₁-C₂₀ alkyl (preferably C₁-C₁₈ alkyl, morepreferably C₁-C₁₂ alkyl, most preferably C₁-C₆ alkyl), C₂-C₂₀ alkenyl(preferably C₂-C₁₈ alkenyl, more preferably C₂-C₂ alkenyl, mostpreferably C₂-C₆ alkenyl), C₂-C₂₀ alkynyl (preferably C₂-C₁₈ alkynyl,more preferably C₂-C₁₂ alkynyl, most preferably C₂-C₆ alkynyl), C₅-C₂₀aryl (preferably C₅-C₁₄ aryl), C₆-C₂₄ alkaryl (preferably C₆-C₁₈alkaryl), and C₆-C₂₄ aralkyl (preferably C₆-C₁₈ aralkyl).

[0060] In addition, the aforementioned functional groups may, if aparticular group permits, be further substituted with one or moreadditional functional groups or with one or more hydrocarbyl moietiessuch as those specifically enumerated above. Analogously, theabove-mentioned hydrocarbyl moieties may be further substituted with oneor more functional groups or additional hydrocarbyl moieties such asthose specifically enumerated.

[0061] By “substantially free of” a particular type of chemical compoundis meant that a composition or product contains less 10 wt. % of thatchemical compound, preferably less than 5 wt. %, more preferably lessthan 1 wt. %, and most preferably less than 0.1 wt. %. For instance, thedepolymerization product herein is “substantially free of” metalcontaminants, including metals per se, metal salts, metallic complexes,metal alloys, and organometallic compounds.

[0062] “Optional” or “optionally” means that the subsequently describedcircumstance may or may not occur, so that the description includesinstances where the circumstance occurs and instances where it does not.For example, the phrase “optionally substituted” means that anon-hydrogen substituent may or may not be present on a given atom, and,thus, the description includes structures wherein a non-hydrogensubstituent is present and structures wherein a non-hydrogen substituentis not present.

[0063] Accordingly, the invention features a method for depolymerizing apolymer having a backbone containing electrophilic linkages. Theelectrophilic linkages may be, for example, ester linkages (CO)—O—),carbonate linkages (—O—(CO)—O)—, urethane linkages (—O—(CO)—NH),substituted urethane linkages (—O—(CO)—NR—, where R is a nonhydrogensubstituent such as alkyl, aryl, alkaryl, or the like), amido linkages(—(CO)—NH—), substituted amido linkages (CO)—NR— where R is as definedpreviously, thioester linkages (—(CO)—S—), sulfonic ester linkages(—S(O)₂—O—), and the like. Other electrophilic linkages that can becleaved using nucleophilic reagents will be known to those of ordinaryskill in the art of organic chemistry and polymer science and/or can bereadily found by reference to the pertinent texts and literature. Thepolymer undergoing depolymerization may be linear or branched, and maybe a homopolymer or copolymer, the latter including random, block,multiblock, and alternating copolymers, terpolymers, and the like.Examples of polymers that can be depolymerized using the methodology ofthe invention include, without limitation:

[0064] poly(alkylene terephthalates) such as fiber-grade PET (ahomopolymer made from monoethylene glycol and terephthalic acid),bottle-grade PET (a copolymer made based on monoethylene glycol,terephthalic acid, and other comonomers such as isophthalic acid,cyclohexene dimethanol, etc.), poly (butylene terephthalate) (PBT), andpoly(hexamethylene terephthalate);

[0065] poly(alkylene adipates) such as poly(ethylene adipate),poly(1,4-butylene adipate), and poly(hexamethylene adipate);

[0066] poly(alkylene suberates) such as poly(ethylene suberate);

[0067] poly(alkylene sebacates) such as poly(ethylene sebacate);

[0068] poly(ε-caprolactone) and poly(β-propiolactone);

[0069] poly(alkylene isophthalates) such as poly(ethylene isophthalate);

[0070] poly(alkylene 2,6-naphthalene-dicarboxylates) such aspoly(ethylene 2,6-naphthalene-dicarboxylate);

[0071] poly(alkylene sulfonyl-4,4′-dibenzoates) such as poly(ethylenesulfonyl-4,4′-dibenzoate);

[0072] poly(p-phenylene alkylene dicarboxylates) such aspoly(p-phenylene ethylene dicarboxylates);

[0073] poly(trans-1,4-cyclohexanediyl alkylene dicarboxylates) such aspoly(trans-1,4-cyclohexanediyl ethylene dicarboxylate);

[0074] poly(1,4-cyclohexane-dimethylene alkylene dicarboxylates) such aspoly(1,4-cyclohexane-dimethylene ethylene dicarboxylate);

[0075] poly([2.2.2]-bicyclooctane-1,4-dimethylene alkylenedicarboxylates) such as poly([2.2.2]-bicyclooctane-1,4-dimethyleneethylene dicarboxylate);

[0076] lactic acid polymers and copolymers such as (S)-polylactide,(R,S)-polylactide, poly(tetramethylglycolide), andpoly(lactide-co-glycolide); and

[0077] polycarbonates of bisphenol A, 3,3′-dimethylbisphenol A,3,3′,5,5′-tetrachlorobisphenol A, 3,3′,5,5′-tetramethylbisphenol A;

[0078] polyamides such as poly(p-phenylene terephthalamide) (Kevlar®);

[0079] poly(alkylene carbonates) such as poly(propylene carbonate);

[0080] polyurethanes such as those available under the tradenamesBaytec® and Bayfil®, from Bayer Corporation; and

[0081] polyurethane/polyester copolymers such as that available underthe tradename Baydar®, from Bayer Corporation.

[0082] Depolymerization of the polymer is carried out, as indicatedabove, in the presence of a nucleophilic reagent and a catalyst.Nucleophilic reagents, as will be appreciated by those of ordinary skillin the art, include monohydric alcohols, diols, polyols, thiols, primaryamines, and the like, and may contain a single nucleophilic moiety ortwo or more nucleophilic moieties, e.g., hydroxyl, sulfhydryl, and/oramino groups. The nucleophilic reagent is selected to correspond to theparticular electrophilic linkages in the polymer backbone, such thatnucleophilic attack at the electrophilic linkage results in cleavage ofthe linkage. For example, a polyester can be cleaved at the esterlinkages within the polymer backbone using an alcohol, preferably aprimary alcohol, most preferably a C₂-C₄ monohydric alcohol such asethanol, isopropanol, and t-butyl alcohol. It will be appreciated thatsuch a reaction cleaves the ester linkages via a transesterificationreaction, as will be illustrated infra.

[0083] The preferred catalysts for the depolymerization reaction arecarbenes and carbene precursors. Carbenes include, for instance,diarylcarbenes, cyclic diaminocarbenes, imidazol-2-ylidenes,1,2,4-triazol-3-ylidenes, 1,3-thiazol-2-ylidenes, acyclicdiaminocarbenes, acyclic aminooxycarbenes, acyclic aminothiocarbenes,cyclic diborylcarbenes, acyclic diborylcarbenes, phosphinosilylcarbenes,phosphinophosphoniocarbenes, sulfenyl-trifluoromethylcarbene, andsulfenyl-pentafluorothiocarbene. See Bourissou et al. (2000), citedsupra. Preferred carbenes are heteroatom-stabilized carbenes andpreferred carbene precursors are precursors to heteroatom-stabilizedcarbenes nitrogen-containing carbenes, with N-heterocyclic carbenes mostpreferred.

[0084] In one embodiment, heteroatom-stabilized carbenes suitable asdepolymerization catalysts herein have the structure of formula (I)wherein the various substituents are as follows:

[0085] E¹ and E² are independently selected from N, NR^(E), O, P,PR^(E), and S, R^(E) is hydrogen, heteroalkyl, or heteroaryl, and x andy are independently zero, 1, or 2, and are selected to correspond to thevalence state of E¹ and E², respectively. When E¹ and E² are other thanO or S, then E¹ and E² may be linked through a linking moiety thatprovides a heterocyclic ring in which E¹ and E² are incorporated asheteroatoms. In the latter case, the heterocyclic ring may be aliphaticor aromatic, and may contain substituents and/or heteroatoms. Generally,such a cyclic group will contain 5 or 6 ring atoms.

[0086] For example, in representative compounds of formula (I):

[0087] (1) E¹ is O or S and x is 1;

[0088] (2) E¹ is N, x is 1, and E¹ is linked to E²;

[0089] (3) E¹ is N, x is 2, and E¹ and E² are not linked;

[0090] (4) E¹ is NR^(E), x is 1, and E¹ and E² are not linked; or

[0091] (5) E¹ is NR^(E), x is zero, and E¹ is linked to E².

[0092] R¹ and R² are independently selected from branched C₃-C₃₀hydrocarbyl, substituted branched C₃-C₃₀ hydrocarbyl,heteroatom-containing branched C₄-C₃₀ hydrocarbyl, substitutedheteroatom-containing branched C₄-C₃₀ hydrocarbyl, cyclic C₅-C₃₀hydrocarbyl, substituted cyclic C₅-C₃₀ hydrocarbyl,heteroatom-containing cyclic C₁-C₃₀ hydrocarbyl, and substitutedheteroatom-containing cyclic C₁-C₃₀ hydrocarbyl. Preferably, at leastone of R¹ and R², and more preferably both R¹ and R², are relativelybulky groups, particularly branched alkyl (including substituted and/orheteroatom-containing alkyl), aryl (including substituted aryl,heteroaryl, and substituted heteroaryl), alkaryl (including substitutedand/or heteroatom-containing aralkyl), and alicyclic. Using suchsterically bulky groups to protect the highly reactive carbene centerhas been found to kinetically stabilize singlet carbenes, which arepreferred reaction catalysts herein. Particular sterically bulky groupsthat are suitable as R¹ and R² are optionally substituted and/orheteroatom-containing C₃-C₁₂ alkyl, tertiary C₄-C₁₂ alkyl, C₅-C₁₂ aryl,C₆-C₁₈ alkaryl, and C₅-C₁₂ alicyclic, with C₅-C₁₂ aryl and C₆-C₁₂alkaryl particularly preferred. The latter substituents are exemplifiedby phenyl optionally substituted with 1 to 3 substituents selected fromlower alkyl, lower alkoxy, and halogen, and thus include, for example,p-methylphenyl, 2,6-dimethylphenyl, and 2,4,6-trimethylphenyl (mesityl).

[0093] L¹ and L² are linkers containing 1 to 6 spacer atoms, and areindependently selected from heteroatoms, substituted heteroatoms,hydrocarbylene, substituted hydrocarbylene, heteroatom-containinghydrocarbylene, and substituted heteroatom-containing hydrocarbylene;and m and n are independently zero or 1, meaning that each of L¹ and L²is optional. Preferred L¹ and L² moieties include, by way of example,alkylene, alkenylene, arylene, aralkylene, any of which may beheteroatom-containing and/or substituted, or L¹ and/or L² may be aheteroatom such as O or S, or a substituted heteroatom such as NH, NR(where R is alkyl, aryl, other hydrocarbyl, etc.), or PR; and

[0094] In one preferred embodiment, E¹ and E² are independently N orNR^(E) and are not linked, such that the carbene is an N-heteroacycliccarbene. In another preferred embodiment, E¹ and E² are N, x and y are1, and E¹ and E² are linked through a linking moiety such that thecarbene is an N-heterocyclic carbene. N-heterocyclic carbenes suitableherein include, without limitation, compounds having the structure offormula (II)

[0095] wherein R¹, R², L¹, L², m, and n are as defined above forcarbenes of formula (I). In carbenes of structural formula (II), L is ahydrocarbylene, substituted hydrocarbylene, heteroatom-containinghydrocarbylene, or substituted heteroatom-containing hydrocarbylenelinker, wherein two or more substituents on adjacent atoms within L maybe linked to form an additional cyclic group. L is a hydrocarbylene,substituted hydrocarbylene, heteroatom-containing hydrocarbylene, orsubstituted heteroatom-containing hydrocarbylene linker, wherein two ormore substituents on adjacent atoms within L may be linked to form anadditional cyclic group. For example, L may be —CR³R⁴—CR⁵R⁶— or—CR³═CR⁵—, wherein R³, R⁴, R⁵, and R⁶ are independently selected fromhydrogen, halogen, C₁-C₁₂ alkyl, or wherein any two of R³, R⁴, R⁵, andR⁶ may be linked together to form a substituted or unsubstituted,saturated or unsaturated ring.

[0096] Accordingly, when L is —CR³R⁴—CR⁵R⁶—or —CR³═CR⁵—, the carbene hasthe structure of formula (II)

[0097] in which q is an optional double bond, s is zero or 1, and t iszero or 1, with the proviso that when q is present, s and t are zero,and when q is absent, s and t are 1.

[0098] Certain carbenes are new chemical compounds and are claimed assuch herein. These are compounds having the structure of formula (I)wherein a heteroatom is directly bound to E¹ and/or E². e.g., with theproviso that a heteroatom is directly bound to E¹, E², or to both E¹ andE², and wherein the carbene may be in the form of a salt (such that itis positively charged and associated with a negatively chargedcounterion). These novel carbenes are those wherein a heteroatom isdirectly bound to E¹ and/or E², and include, solely by way of example,carbenes of formula (I) wherein E¹ and/or E² is NR^(E) ^(and R) ^(E) isa heteroalkyl or heteroaryl group such as an alkoxy, alkylthio, aryloxy,arylthio, aralkoxy, or aralkylthio moiety. Other such carbenes are thosewherein x and/or y is at least 1, and L¹ and/or L² is heteroalkyl,heteroaryl, or the like, wherein the heteroatom within L¹ and/or L² isdirectly bound to E¹ and/or E², respectively.

[0099] Representative of such novel carbenes are compounds of formula(I) wherein E¹ is NR^(E), and R^(E) is alkoxy, substituted alkoxy,aryloxy, substituted aryloxy, aralkoxy, or substituted aralkoxy. Apreferred subset of such carbenes are those wherein E² is N, x is zero,y is 1, and E¹ and E² are linked through a substituted or unsubstitutedlower alkylene or lower alkenylene linkage. A more preferred subset ofsuch carbenes are those wherein R^(E) is lower alkoxy or monocyclicaryl-substituted lower alkoxy, E¹ and E² are linked through a moiety—CR³R⁴—CR⁵R⁶— or —CR³═CR⁵—, wherein R³, R⁴, R⁵, and R⁶ are independentlyselected from hydrogen, halogen, and C₁-C₁₂ alkyl, n is 1, L² is loweralkylene, and R² is monocyclic aryl or substituted monocyclic aryl.Examples 8-11 describe syntheses of representative compounds within thisgroup.

[0100] As indicated previously, suitable catalysts for the presentdepolymerization reaction are also precursors to carbenes, preferablyprecursors to N-heterocyclic and N-heteroacyclic carbenes. In oneembodiment, the precursor is a tri-substituted methane compound havingthe structure of formula (PI)

[0101] wherein E¹, E², x, y, R¹, R², L¹, L², m, and n are as defined forcarbenes of structural formula (I), and wherein R⁷ is selected fromalkyl, heteroalkyl, aryl, heteroaryl, aralkyl, or heteroaralkyl, and issubstituted with at least one electron-withdrawing substituent such as

[0102] fluoro, fluoroalkyl (including perfluoroalkyl), chloro, nitro,acytyl. It will be appreciated that the foregoing list is not exhaustiveand that any electron-withdrawing group may serve as a substituentproviding that the group does not cause unwanted interaction of thecatalyst with other components of the depolymerization mixture oradversely affect the depolymerization reaction in any way. Specificexamples of R⁷ groups thus include p-nitrophenyl, 2,4-dinitrobenzyl,1,1,2,2-tetrafluoroethyl, pentafluorophenyl, and the like.

[0103] Catalysts of formula (PI) are new chemical entities.Representative syntheses of such compounds are described in Examples 13and 14 herein. As may be deduced from those examples, compounds offormula (PI) wherein E¹ and E² are N may be synthesized from thecorresponding diamine and an appropriately substituted aldehyde.

[0104] Another carbene precursor useful as a catalyst in the presentdepolymerization reaction has the structure of formula (PII)

[0105] wherein E¹, E², x, y, R¹, R², L¹, L², m, and n are as defined forcarbenes of structural formula (I), M is a metal, e.g., gold, silver,other main group metals, or transition metals, with Ag, Cu, Ni, Co, andFe generally preferred, and Ln is a ligand, generally an anionic orneutral ligand that may or may not be the same as -E¹-[(L¹)_(m)-R¹]_(x)or -E²-[(L²)_(n)-R²]_(y). Generally, carbene precursors of formula (PII)can be synthesized from a carbene salt and a metal oxide; see, e.g., thesynthesis described in detail in Example 12.

[0106] Still another carbene precursor suitable as a depolymerizationcatalyst herein is a tetrasubstituted olefin having the structure offormula (PIII)

[0107] wherein: E¹, E², x, y, R¹, R², L¹, L², m, and n are defined asfor carbenes of structural formula (I); E³ and E⁴ are defined as for E¹and E²; v and w are defined as for x and y; R⁸ and R⁹ are defined as forR¹ and R²; L³ and L⁴ are defined as for L¹ and L 2; and h and k aredefined as for m and n. These olefins are readily formed fromN,N-diaryl- and N,N-dialkyl-N-heterocyclic carbene salts and a strongbase, typically an inorganic base such as a metal alkoxide.

[0108] As with the carbenes per se, those catalyst precursors having thestructure of formula (PII) or (PIII) in which a heteroatom is directlybound to an “E” moiety, i.e., to E¹, E², E³, and/or E⁴, are new chemicalentities. Preferred such precursors are those wherein the “E” moietiesare NR^(E) or linked N atoms, and the directly bound heteroatom withinR^(E) is oxygen or sulfur.

[0109] The depolymerization reaction may be carried out in an inertatmosphere by dissolving a catalytically effective amount of theselected catalyst in a solvent, combining the polymer and the catalystsolution, and then adding the nucleophilic reagent. In a particularlypreferred embodiment, however, the polymer, the nucleophilic reagent,and the catalyst (e.g., a carbene or a carbene precursor) are combinedand dissolved in a suitable solvent, and depolymerization thus occurs ina one-step reaction.

[0110] Preferably, the reaction mixture is agitated (e.g., stirred), andthe progress of the reaction can be monitored by standard techniques,although visual inspection is generally sufficient, insofar as atransparent reaction mixture indicates that the polymer has degraded toan extent sufficient to allow all degradation products to go intosolution. Examples of solvents that may be used in the polymerizationreaction include organic, protic, or aqueous solvents that are inertunder the depolymerization conditions, such as aromatic hydrocarbons,chlorinated hydrocarbons, ethers, aliphatic hydrocarbons, or mixturesthereof. Preferred solvents include toluene, methylene chloride,tetrahydrofuran, methyl t-butyl ether, Isopar, gasoline, and mixturesthereof. Supercritical fluids may also be used as solvents, with carbondioxide representing one such solvent. Reaction temperatures are in therange of about 0° C. to about 100° C., typically at most 80° C.,preferably 60° C. or lower, and most preferably 30° C. or less, and thereaction time will generally be in the range of about 12 to 24 hours.Pressures range from atmospheric to pressures typically used inconjunction with supercritical fluids, with the preferred pressure beingatmospheric.

[0111] It is to be understood that while the invention has beendescribed in conjunction with the preferred specific embodimentsthereof, that the foregoing description as well as the examples thatfollow are intended to illustrate and not limit the scope of theinvention. Other aspects, advantages and modifications within the scopeof the invention will be apparent to those skilled in the art to whichthe invention pertains.

[0112] All patents, patent applications, and publications mentionedherein are hereby incorporated by reference in their entireties.

EXPERIMENTAL

[0113] General Procedures. ¹H and ¹³C NMR spectra were recorded on aBruke-Avance (400 MHz for ¹H and 100 MHz for ¹³C). All NMR spectra wererecorded in CDCl₃. Materials. Solvents were obtained from Sigma-Aldrichand purified by distillation. Other reagents were obtained commerciallyor synthesized as follows: poly(propylene carbonate), poly(bisphenol Acarbonate), poly(1,4-butylene adipate), 1-ethyl-3-methyl-1-H-imidazoliumchloride, ethylene glycol, butane-2,3-dione monooxime, ammoniumhexafluorophosphate, pentafluorobenzaldehyde, and mesityl diamine,obtained from Sigma-Aldrich;1,3-(2,4,6-trimethylphenyl)imidazol-2-ylidene, synthesized according tothe method of Arduengo et al. (1999) Tetrahedron 55:14523; N,N-diphenylimidazoline, chloride salt, synthesized according to the method ofWanzlick et al. (1961) Angew. Chem. 73:493 and Wanzlick et al. (1962)Angew. Chem. 74:128, and Wanzlick et al. (1963) Chem. Ber. 96:3024;1,3,5-tribenzyl-[1,3,5]triazinane, synthesized according to the methodof Arduengo et al. (1992) J. Am. Chem. Soc. 114:5530, cited supra.

Example 1

[0114] Depolymerization of Poly(propylene carbonate) (M_(w)=50,000) withisolated carbene: 7 mg (0.02 mmol) of1,3-(2,4,6-trimethylphenyl)imidazol-2-ylidene dissolved in toluene (0.6mL), was added to a stirred mixture of 0.5 g of poly(propylenecarbonate) in toluene (10 mL), under N₂. After stirring for 5 minutes atroom temperature, 2 mL of methanol were added to the reaction mixtureand the temperature was brought to 80° C. Stirring was continued for 3hours followed by the evaporation of the solvent in vacuo. The ¹H and¹³C NMR spectra showed the presence of a single monomer,4-methyl-[1,3]-dioxolan-2-one. However, there were 4 peaks in the GC-MS.

[0115] GC-MS:

[0116] a) m/z (5%) 5.099 min=106 (42), 103 (5), 91 (100), 77 (8), 65(8), 51 (8)

[0117] b) m/z (5%) 5.219 min=106 (60), 105 (30), 103 (8), 91 (100), 77(8), 65 (5), 51 (5)

[0118] c) m/z (85%) 6.750 min=102 (15), 87 (40), 58 (20), 57 (100).Major product.

[0119] d) m/z (5%) 9.030 min=136(10), 135 (100), 134 (70), 120 (85), 117(8), 103 (5), 91 (14), 77 (10), 65 (5).

[0120]¹H NMR:1.4 (d, 3H), 3.9 (t, 1H), 4.5 (t, 1H), 4.8 (m, 1H).

[0121]¹³C NMR: 18.96, 70.42, 73.43, 154.88

Example 2

[0122] Depolymerization of Poly(Bisphenol A carbonate) (M_(w)=65,000)with isolated carbene: 7 mg (0.02 mmol) of1,3-(2,4,6-trimethylphenyl)imidazol-2-ylidine dissolved in toluene (1mL), was added to a stirred mixture of 0.5 g of poly(bisphenol Acarbonate) in toluene (10 mL), under N₂. After stirring for 5 minutes atroom temperature, 2 mL of methanol were added to the reaction mixture.The temperature was brought to 80° C. and stirring was continued for 18hours followed by the evaporation of the solvent in vacuo. The ¹H and¹³C NMR spectra showed the presence of two compounds identified as,bisphenol A and carbonic acid4-[1-hydroxy-phenyl)-1-methyl-ethyl]-phenyl ester4-[1-(4-methoxy-phenyl)-1-methyl-ethyl]phenyl ester. However, GC-MSindicated 4 peaks.

[0123] GC-MS:

[0124] a) m/z (5%) 5.107 min=106 (40), 103 (5), 91 (100), 77 (8), 65(8), 51 (8)

[0125] b) m/z (5%) 5.210 min=106 (60), 105 (30), 103 (8), 91 (100), 77(8), 65 (5), 51 (5)

[0126] c) m/z (60%)14.301 min=228 (30), 213 (100), 119 (15), 91 (10).Major product

[0127] d) m/z (30%) 16.016 min=495 (30), 333 (10), 319 (20), 299 (5),281 (5), 259 (25), 239 (38), 197 (40), 181 (12), 151 (12), 135 (100),119 (10), 91 (10).

[0128]¹H NMR: 1.6-1.8 (m), 2,4 (s), 3.96 (s), 6.7-6.8 (t), 7.0-7.3 (m).

Example 3

[0129] Depolymerization of Poly(1,4-butylene adipate) (M_(w)=12,000)with isolated carbene: 0.006 g (0.02 mmol) of1,3-(2,4,6-trimethylphenyl)imidazol-2-ylidine dissolved in toluene (1mL), was added to a stirred mixture of 1.0 g of poly(1,4-butyleneadipate) in toluene (10 mL), under N₂. After stirring for 5 minutes atroom temperature, 2 mL of methanol were added to the reaction mixture.The temperature was brought to 80° C. and stirring was continued for 6hours followed by the evaporation of the solvent in vacuo. The ¹H and¹³C NMR showed the presence of a single product, and the GC-MS showedtwo products.

[0130] GC-MS:

[0131] a) m/z (95%) 5.099 min=143 (80), 142 (20), 115 (20), 114 (100),111 (70), 101 (65), 87 (12), 83 (25), 82 (12), 74 (36), 73 (26), 69(10), 59 (72), 55 (60). Major product.

[0132] b) m/z (5%) 12.199 min=201 (4), 161 (6), 143 (100), 129 (32), 116(12), 115 (25), 111 (70), 101 (12), 87 (10), 83 (15), 73 (34), 71 (12),59 (14), 55 (42).

[0133]¹H NMR:1.67 (m), 2.32 (s), 4.08 (s).

[0134]¹³C NMR: 24.26, 25.18, 33.74, 63.75, 173.23

Example 4

[0135] Depolymerization of Poly(propylene carbonate) (M=50,000) within-situ carbene: To a mixture of 7 mg (0.047 mmol) of1-ethyl-3-methyl-1-H-imidazolium chloride in tetrahydrofuran (THF) wasadded 4 mg (0.038 mmol) of potassium t-butoxide (t-BOK), under N₂. After30 min stirring, 0.1 mL of the reaction mixture was transferred to aflask that was charged with 0.5 g of poly(propylene carbonate) in 10 mLof THF. The reaction mixture was stirred for 10 min at room temperaturefollowed by the addition of 2 mL of methanol. Stirring was continued atroom temperature for 3 hours. Solvent was removed and the ¹H and ¹³C NMRspectra showed the presence of a single product,4-methyl-[1,3]-dioxolan-2-one. However, before the removal of thesolvent the GC-MS of the crude reaction mixture showed 6 differentcompounds.

[0136] GC-MS:

[0137] a) m/z (15%) 6.268 min=119 (4), 90 (100), 75 (4), 59 (25).

[0138] b) m/z (5%) 6.451 min=104 (40), 103 (30), 90 (5), 77 (5), 59(100), 58 (10), 57 (10).

[0139] c) m/z (70%) 6.879 min=102 (10), 87 (25), 58 (14), 57 (100).Major product.

[0140] d) m/z (1%) 7.565 min=103 (40), 89 (5), 59 (100), 58 (5), 57 (8).

[0141] e) m/z (4%) 8.502 min=207 (14), 133 (10), 103 (35), 90 (10), 89(10), 59 (100), 58 (12), 57 (14).

[0142] f) m/z (5%) 8.936 min=148 (8), 118 (8), 117 (15), 103 (20), 77(60), 72 (8), 59 (100), 58 (5), 57 (5).

[0143]¹H NMR:1.4 (d, 3H), 3.9 (t, 1H), 4.5 (t, 1H), 4.8 (m, 1H).

[0144]¹³C NMR: 18.96, 70.42, 73.43, 154.88

Example 5

[0145] Depolymerization of Poly(bisphenol A carbonate) (M_(w)=65,000)with in situ carbene: To a mixture of 7 mg (0.047 mmol) of1-ethyl-3-methyl-1-H-imidazolium chloride in THF (1 mL) was added 4 mg(0.038 mmol) of t-BOK, under N₂. After 30 min, stirring 0.1 mL of thereaction mixture was transferred to a flask that was charged with 0.5 gof poly(bisphenol A carbonate) in 10 mL of THF. The reaction mixture wasstirred for 10 min at room temperature followed by the addition of 2 mLof methanol. Stirring was continued at room temperature for 3 hours. Thesolvent was removed in vacuo and the ¹H,

[0146]¹³C NMR and GC-MS spectra showed a mixture of monomer andoligomers, where the major product was bisphenol A.

[0147] GC-MS:

[0148] a) m/z (10%) 12.754 min=212 (30), 198 (20), 197 (100), 182 (10),181 (10), 179 (10), 178 (10), 165 (8), 152 (8), 135 (10), 119 (12), 103(15), 91 (12), 77 (10), 65 (5).

[0149] b) m/z (5%) 13.674 min=282 (5), 281 (10), 255 (8), 229 (10), 228(40), 214 (20), 213 (100), 208 (30), 197 (30), 191 (5), 181 (5), 179(5), 165 (10), 152 (8), 135 (25), 134 (25), 133 (5), 120 (5), 119 (50),115 (10), 103 (10), 99 (5), 97 (5), 96 (5), 91 (30), 79 (5), 77 (10), 65(8).

[0150] c) m/z (35%) 14.286 min=228 (34), 214 (20), 213 (100), 197 (5),165 (5), 135 (5), 119 (20), 107 (5), 91 (10), 77 (5), 65 (5). MajorProduct.

[0151] d) m/z (35%) 15.189 min=286 (20), 272 (15), 271 (100), 227 (5),212 (5), 197 (3), 183 (2), 169 (3), 133 (3), 119 (5).

[0152] e) m/z (10%) 15.983 min=344 (20), 330 (20), 329 (100), 285 (5),269 (3), 226 (3), 211 (2), 183 (3), 165 (3), 153 (2), 133 (6), 121 (2),91 (2), 77 (1), 59 (3).

Example 6

[0153]

[0154] Depolymerization of PET according to the above scheme: 20 mg oft-BOK and 45 mg of N,N-diphenyl imidazoline, chloride salt, were placedin a vial with 2 mL THF and stirred for 15 minutes. Ethylene glycol (2.3g) and PET (0.25 g) (pellets obtained from Aldrich dissolved in CHCl₃and trifluoroacetic acid and precipitated with methanol to form a whitepowder) were combined to form a PET slurry. The catalyst was added tothe slurry with approximately 5 additional mL THF. After 2 hours, thesolution became more transparent, indicating dissolution of thecomponents of the depolymerization mixture. The admixture was stirredovernight, yielding a completely clear solution the following day theTHF was removed, yielding 225 mg of white solid. ¹H NMR ¹³C NMR, andGC-MS were all consistent with bis(hydroxy ethylene) terephthalate.

Example 7

[0155]

[0156] Depolymerization of PET according to the above scheme: 25 mg of1,3-dimethyl imidazole, iodide salt, and 11 mg of t-BOK were placed in avial with 2 mL of THF and stirred for 15 min. Methanol (3.11 g) and PET(308 mg, as in Example 6) were combined with 5 mL of THF to form aninsoluble mixture. The catalyst mixture was filtered into thePET/methanol mixture. After 1 hour, there was a noticeable increase intransparency. After 14 hours, the solution was completely homogeneousand clear. The solvent was removed by rotary evaporation to yield awhite crystalline product (250 mg). ¹H NMR indicated complete conversionto dimethyl terephthalate.

[0157] Examples 6 and 7 may be better understood by reference to thesynthetic route used to prepare the PET and the possibledepolymerization products obtained therefrom. The PET obtained in eachexample was prepared by synthesis according to a two-steptransesterification process from dimethyl teraphthalate (DMT) and excessethylene glycol (EO) in the presence of a metal alkanoate or acetate ofcalcium, zinc, manganese, titanium etc. The first step generatesbis(hydroxy ethylene) teraphthalate (BHET) with the elimination ofmethanol and the excess EO. The BHET is heated, generally in thepresence of a transesterification catalyst, to generate high polymer.This process is generally accomplished in a vented extruder to removethe polycondensate (EO) and generate the desired thermoformed objectfrom a low viscosity precursor. The reaction takes place according tothe following scheme:

[0158] The different options for chemical recycling are regeneration ofthe base monomers (DMT and EG), glycolysis of PET back to BHET,decomposition of PET with propylene glycol and reaction of thedegradation product with maleic anhydride to form “unsaturatedpolyesters” for fiber reinforced composites and decomposition withglycols, followed by reaction with dicarboxylic acids to produce polyolsfor urethane foam and elastomers.

[0159] In Example 7, PET powder was slurried in a THF/methanol solventmixture. N-heterocyclic carbene (3-5 mol %), generated in situ, wasadded and within approximately 3 hours the PET went into solution.Anaylsis of the degradation product indicated quantitative consumptionof PET and depolymerization via transesterification to EO and DMT. TheDMT is readily recovered by recrystallization, while EO can be recoveredby distillation (FIG. 1). Alternatively, and as established in Example6, if EO is used as the alcohol (˜50 to 200 mol % excess) in the THFslurry, the depolymerization product is BHET, which is the mostdesirable and can be directly recycled via conventional methods to PET(FIG. 2). The N-heterocyclic carbene catalyst platform is extremelypowerful, as the nature of the substituents has a pronounced effect oncatalyst stability and activity towards different substrates.

[0160] The PET depolymerization reactions of Examples 6 and 7 areillustrated schematically below.

[0161] The following Examples 8-11 describe synthesis of new carbeneprecursors as illustrated in the following scheme:

Example 8

[0162] 3-Benzyl-1-methoxy-4,5-dimethylimidazolium iodide (2): Methyliodide (0.5 mL, 7.8 mmol) was added via syringe to a solution ofimidazole-N-oxide 1 (1.0 g, 4.9 mmol) in ca. 20 mL of CHCl₃ (compound 1was prepared from 1,3,5-tribenzyl-[1,3,5]triazinane and butane-2,3-dionemonooxime using the procedure of Arduengo et al. (1992), supra.) Theresulting mixture was stirred at room temperature overnight. Removal ofthe volatiles in vacuo afforded a thick yellow oil of suitable purity inan undetermined yield.

[0163]¹H-NMR (6, CDCl₃): 10.32 (s, 1H, N—CH—N); 7.39 (m, 5H, C₆H₅); 5.56(s, 2H, NCH₂); 4.38 (s, 3H, OCH₃); 2.27 (s, 3H, CH₃); 2.20 (s, 3H, CH₃).

Example 9

[0164] 3-Benzyl-1-methoxy-4,5-dimethylimidazolium hexafluorophosphate(3): Crude iodide 2 was taken up in deionized (DI) water, whichseparated the product from small amounts of a dark, insoluble residue.The water solution was decanted to a second flask and a solution ofammonium hexafluorophosphate (950 mg, ca. 5.8 mmol) in 10 mL of DI waterwas added in portions. An oil separated during the addition, and thesupernatant solution was decanted out. The oil was crushed in cold (0°C.), and subsequently recrystallized in methanol. Yield: 1.3 g (73% from1). ¹H-NMR (6, CDCl₃): 8.67 (s, 1H, N—CH—N); 7.39 (m, 3H, C₆H₅); 7.29(d, 2H, C₆H₅); 5.24 (s, 2H, NCH₂); 4.21 (s, 3H, OCH₃); 2.27 (s, 3H,CH₃); 2.17 (s, 3H, CH₃).

Example 10

[0165] 1-Benzyloxy-3-benzyl-4,5-dimethylimidazolium bromide (4): Benzylbromide (1.2 mL, ca. 10 mmol) was added via syringe to a refluxingsuspension of 1 (1.0 g, 5.0 mmol) in dry benzene. A dark orange oilseparated after refluxing for 6 h, and cooling to room temperature. Thesupernatant was decanted and the remaining oil was dried under vacuumovernight, which caused the product to solidify. The solid mass wascrushed in pentane, filtered and dried under vacuum. Yield: 1.34 g(63%). ¹H-NMR (6, CDCl₃): 11.04 (s, 1H, N—CH—N); 7.6-7.2 (ov. m, 10H,2×C₆H₅); 5.59, 5.58 (s+s, N—CH₂, O—CH₂); 2.09, 1.94 (s, 3H, CH₃, CH₃).¹³C-NMR (6, CDCl₃): 132.8 (OCH₂-C₆H₅); 132.5 (NCN); 131.5(NCH₂-^(i)C₆H₅); 130.6, 130.3, 129.2, 129.0, 129.0, 128.9, 128.0(^(omp)C₆H₅); 124.8; 124.1 (NCCN); 83.9 (OCH₂); 51.2 (NCH₂); 8.89 (CH₃);7.11 (CH₃).

Example 11

[0166] 3-Benzyl-1-benzyloxy-4,5-dimethylimidazolium hexafluorophosphate(5): A batch of crude bromide 4 (still as an oil before drying undervacuum) was dissolved in DI water and extracted with hexanes. Theaqueous layer was separated and a solution of ammoniumhexafluorophosphate (ca. 1.3 equiv.) was added dropwise with constantstirring. The yellow oil deposited on the walls of the flask wasdissolved in warm methanol and a few drops of hexanes were added.Cooling to room temperature afforded off-white crystals of pure 5, whichwere rinsed with pentane and dried under vacuum. Yield: (82% from 1).¹H-NMR (δ, CDCl₃): 8.42 (s, 1H, N—CH—N); 7.45-7.35, 7.18 (ov.m, C₆H₅);5.31, 5.20 (s+s, N—CH₂, O—CH₂); 2.13 (s, 3H, CH₃); 2.05 (s, 3H, CH₃).

Example 12

[0167]

[0168] Bis(1-Benzyloxy-3-benzyl-4,5-dimethylimidazolylidene)silver(I)dibromoargentate (6). The carbene precursor 6 was prepared as follows: Amixture of silver oxide (128 mg, 0.55 mmol) and imidazolium bromide 4(396 mg, 1.06 mmol) was taken up in dry CH₂Cl₂ and stirred at roomtemperature for 90 minutes. The dark orange suspension was filteredthrough a pad of celite and evaporated to dryness, yielding an orangepowder. Crystallization from THF afforded a white powder (2 crops).Yield: 291 mg (57%).

[0169]¹H-NMR (δ, CD₂Cl₂): 7.47-7.32 (ov. m, 10H, 2×C₆H₅); 5.23, 5.22(s+s, NCH₂, OCH₂); 2.01, 1.95 (s, 3H+3H, CH₃, CH₃). ¹³C-NMR (δ, CD₂Cl₂):136.2 (NCN); 133.3 (OCH₂—^(i)C₆H₅); 130.8 (NCH₂—^(i)C₆H₅); 130.7, 130.0;129.3, 129.3, 128.5, 127.1, 123.9 (^(omp)C₆H₅+NCCN); 82.6 (OCH₂); 54.0(NCH₂); 9.4 (CH₃); 7.8 (CH₃). Anal. Found: C, 47.56; H, 4.26; N, 5.79%.Calc. for C₃₈H₄₀Ag₂Br₂N₄O₂: C, 47.53; H, 4.20; N, 5.83%.

[0170] Examples 13 and 14 describe preparation of additional carbeneprecursors from N,N-diaryl-substituted diamines as illustrated in theschemes below.

Example 13

[0171]

[0172] Synthesis of carbene precursor 7(2-pentafluorophenyl-1,3-diphenyl-imidazolidine): 200 mg (0.94 mmol,FW=212.12) N,N′-diphenyl-ethane-1,2-diamine was placed in a vial anddissolved in 5 mL CH₂Cl₂. A catalytic amount of p-toluenesulfonic acidand 50 mg of Na₂SO₄ were added, followed by 230 mg (0.94 mmol,FW=196.07) of pentafluorobenzaldehyde. The mixture was stirred for 8 h.The Na₂SO₄ was filtered off and solvent was removed under reducedpressure to yield a light brown powder 395 mg (FW=436.2), 96% yield. ¹HNMR: (400 MHz, CDCl₃, 25° C.) δ=3.7-3.9 (m, 2H), 3.9-4.1 (m, 2H), 6.5(s, 1H), 6.7-6.8 (m, 2H), 6.8-6.9 (m, 1H), 7.2-7.5 (m, 2H). ¹⁹F NMR:δ=−143.2 (s br, 2F), −153.7-−153.8 (m, 1F), 161.7-−161.8 (m, 2F).

Example 14

[0173]

[0174] Synthesis of carbene precursor 8(2-pentafluorophenyl-1,3-bis-(2,4,6-trimethyl-phenyl)-imidazolidine):Mesityldiamine (512 mg, 1.7 mmol) was placed into a vial, equipped witha stirbar, with pentafluorobenzaldehyde (340 mg, 1.7 mmol). Glacialacetic acid (5 mL) was added and the reaction was stirred at roomtemperature for 24 h. The acetic acid was removed under reduced pressureand the product was washed several times with cold methanol to affordthe product as a white crystalline solid (543 mg, 65%). ¹H NMR: (400MHz, CDCl₃, 25° C.) δ: 2.2 (s, 12H), 2.3 (s, 6H), 3.5-3.6 (m, 2H),3.9-3.4 (m, 2H), 6.4 (s, 1H), 6.9 (s, 4H). ¹⁹F NMR: −136.3-−136.4 (m,1F), −148.6-−148.7 (m, 1F), −155.8-−155.9 (m, 1F), −163.0-−163.3 (m,2F).

We claim:
 1. A method for depolymerizing a polymer having a backbonecontaining electrophilic linkages, comprising contacting the polymerwith a nucleophilic reagent and a catalyst that yields depolymerizationproducts that are substantially free of metal contaminants.
 2. Themethod of claim 1, wherein the electrophilic linkages are independentlyselected from ester linkages, carbonate linkages, urethane linkages,substituted urethane linkages, phosphate linkages, amido linkages,substituted amido linkages, thioester linkages, sulfonate esterlinkages, and combinations thereof.
 3. The method of claim 2, wherein atleast some of the electrophilic linkages are ester linkages, such thatthe polymer is a polyester.
 4. The method of claim 3, wherein all of theelectrophilic linkages are ester linkages, such that the polyester is ahomopolymer.
 5. The method of claim 3, wherein at least some of theelectrophilic linkages are other than ester linkages, such that thepolyester is a copolymer.
 6. The method of claim 3, wherein thenucleophilic reagent is a compound containing at least one nucleophilicmoiety selected from hydroxyl groups, amino groups, and sulfhydrylgroups.
 7. The method of claim 6, wherein the compound contains onenucleophilic moiety.
 8. The method of claim 7, wherein the nucleophilicmoiety is a hydroxyl group.
 9. The method of claim 6, wherein thecompound contains two nucleophilic moieties.
 10. The method of claim 9,wherein the nucleophilic moieties are hydroxyl groups.
 11. The method ofclaim 1, wherein the catalyst is selected from carbenes, carbeneprecursors, and combinations thereof.
 12. The method of claim 11,wherein the catalyst is a carbene.
 13. The method of claim 12, whereinthe carbene has the structure of formula (I)

wherein: E¹ and E² are independently selected from N, NR^(E), O, P,PR^(E), and S, R^(E) is hydrogen, heteroalkyl, or heteroaryl, x and yare independently zero, 1, or 2, and are selected to correspond to thevalence state of E¹ and E², respectively, and wherein when E¹ and E² areother than O or S, then E¹ and E² may be linked through a linking moietythat provides a heterocyclic ring in which E¹ and E² are incorporated asheteroatoms; R¹ and R² are independently selected from branched C₃-C₃₀hydrocarbyl, substituted branched C₃-C₃₀ hydrocarbyl,heteroatom-containing branched C₄-C₃₀ hydrocarbyl, substitutedheteroatom-containing branched C₄-C₃₀ hydrocarbyl, cyclic C₅-C₃₀hydrocarbyl, substituted cyclic C₅-C₃₀ hydrocarbyl,heteroatom-containing cyclic C₁-C₃₀ hydrocarbyl, and substitutedheteroatom-containing cyclic C₅-C₃₀ hydrocarbyl; L¹ and L² are linkerscontaining 1 to 6 spacer atoms, and are independently selected fromheteroatoms, substituted heteroatoms, hydrocarbylene, substitutedhydrocarbylene, heteroatom-containing hydrocarbylene, and substitutedheteroatom-containing hydrocarbylene; and m and n are independently zeroor
 1. 14. The method of claim 13, wherein E¹ and E² are N.
 15. Themethod of claim 14, wherein x and y are 1, and E¹ and E² are linkedthrough a linking moiety such that the carbene is an N-heterocycliccarbene.
 16. The method of claim 15, wherein the N-heterocyclic carbenehas the structure of formula (II)

wherein: R¹ and R² are independently selected from branched C₃-C₃₀hydrocarbyl, substituted branched C₃-C₃₀ hydrocarbyl,heteroatom-containing branched C₄-C₃₀ hydrocarbyl, substitutedheteroatom-containing branched C₄-C₃₀ hydrocarbyl, cyclic C₅-C₃₀hydrocarbyl, substituted cyclic C₅-C₃₀ hydrocarbyl,heteroatom-containing cyclic C₁-C₃₀ hydrocarbyl, and substitutedheteroatom-containing cyclic C₁-C₃₀ hydrocarbyl; L is the linkingmoiety, and is selected from a hydrocarbylene, substitutedhydrocarbylene, heteroatom-containing hydrocarbylene, and substitutedheteroatom-containing hydrocarbylene linker, wherein two or moresubstituents on adjacent atoms within L may be linked to form anadditional cyclic group; L¹ and L² are lower alkylene; and m and n areindependently zero or
 1. 17. The method of claim 16, wherein: R¹ and R²are independently selected from secondary C₃-C₁₂ alkyl, tertiary C₄-C₁₂alkyl, C₅-C₁₂ aryl, substituted C₅-C₁₂ aryl, C₆-C₁₈ alkaryl, substitutedC₆-C₁₈ alkaryl, C₅-C₁₂ alicyclic, and substituted C₅-C₁₂ alicyclic; andL is —CR³R⁴—CR⁵R⁶—or —CR³═CR⁵—, wherein R³, R⁴, R⁵, and R⁶ areindependently selected from hydrogen, halogen, C₁-C₁₂ alkyl, or whereinany two of R³, R⁴, R⁵, and R⁶ may be linked together to form asubstituted or unsubstituted, saturated or unsaturated ring, such thatthe N-heterocyclic carbene has the structure of formula (III)

in which q is an optional double bond.
 18. The method of claim 17,wherein: R¹ and R² are independently selected from C₅-C₁₂ aryl, mono-,di, and tri-lower alkyl-substituted C₅-C₁₂ aryl, C₆-C₁₂ alkaryl, andmono-, di, and tri-lower alkyl-substituted C₆-C₁₂ alkaryl; m and n arezero; and R³ and R⁴ are hydrogen.
 19. The method of claim 13, wherein E¹and E² are independently N or NR^(E) and are not linked, such that thecarbene is an N-heteroacyclic carbene.
 20. The method of claim 13,wherein E¹ is NR^(E).
 21. The method of claim 20, wherein: R^(E) isalkoxy, substituted alkoxy, aryloxy, substituted aryloxy, aralkoxy, orsubstituted aralkoxy; E² is N; x is zero; y is 1; and E¹ and E² arelinked through a substituted or unsubstituted lower alkylene or loweralkenylene linkage.
 22. The method of claim 21, wherein: R^(E) is loweralkoxy or monocyclic aryl-substituted lower alkoxy; E¹ and E² are linkedthrough a moiety —CR³R⁴—CR⁵R⁶— or —CR³═CR⁵—, wherein R³, R⁴, R⁵, and R⁶are independently selected from hydrogen, halogen, and C₁-C₁₂ alkyl; nis 1; L² is lower alkylene; and R² is monocyclic aryl or substitutedmonocyclic aryl.
 23. The method of claim 11, wherein the catalyst is acarbene precursor.
 24. The method of claim 23, wherein the carbeneprecursor has the structure of formula (PI)

wherein: E¹ and E² are independently selected from N, NR^(E), O, P,PR^(E), and S, R^(E) is hydrogen, heteroalkyl, or heteroaryl, x and yare independently zero, 1, or 2, and are selected to correspond to thevalence state of E¹ and E², respectively, and wherein when E¹ and E² areother than O or S, then E and E² may be linked through a linking moietythat provides a heterocyclic ring in which E¹ and E² are incorporated asheteroatoms; R¹ and R² are independently selected from branched C₃-C₃₀hydrocarbyl, substituted branched C₃-C₃₀ hydrocarbyl,heteroatom-containing branched C₄-C₃₀ hydrocarbyl, substitutedheteroatom-containing branched C₄-C₃₀ hydrocarbyl, cyclic C₅-C₃₀hydrocarbyl, substituted cyclic C₅-C₃₀ hydrocarbyl,heteroatom-containing cyclic C₁-C₃₀ hydrocarbyl, and substitutedheteroatom-containing cyclic C₁-C₃₀ hydrocarbyl; L¹ and L² are linkerscontaining 1 to 6 spacer atoms, and are independently selected fromheteroatoms, substituted heteroatoms, hydrocarbylene, substitutedhydrocarbylene, heteroatom-containing hydrocarbylene, and substitutedheteroatom-containing hydrocarbylene; m and n are independently zero or1; and R⁷ is selected from alkyl, heteroalkyl, aryl, heteroaryl,aralkyl, or heteroaralkyl, substituted with at least oneelectron-withdrawing substituent, and further wherein said contacting iscarried out in the presence of a base.
 25. The method of claim 23,wherein the carbene precursor has the structure of formula (PII)

wherein: E¹ and E² are independently selected from N, NR^(E), O, P,PR^(E), and S, R^(E) is hydrogen, heteroalkyl, or heteroaryl, x and yare independently zero, 1, or 2, and are selected to correspond to thevalence state of E¹ and E², respectively, and wherein when E¹ and E² areother than O or S, then E¹ and E² may be linked through a linking moietythat provides a heterocyclic ring in which E¹ and E² are incorporated asheteroatoms; R¹ and R² are independently selected from branched C₃-C₃₀hydrocarbyl, substituted branched C₃-C₃₀ hydrocarbyl,heteroatom-containing branched C₄-C₃₀ hydrocarbyl, substituted heteroatom-containing branched C₄-C₃₀ hydrocarbyl, cyclic C₅-C₃₀ hydrocarbyl,substituted cyclic C₅-C₃₀ hydrocarbyl, heteroatom-containing cyclicC₁-C₃₀ hydrocarbyl, and substituted heteroatom-containing cyclic C₁-C₃₀hydrocarbyl; L and L² are linkers containing 1 to 6 spacer atoms, andare independently selected from heteroatoms, substituted heteroatoms,hydrocarbylene, substituted hydrocarbylene, heteroatom-containinghydrocarbylene, and substituted heteroatom-containing hydrocarbylene; mand n are independently zero or 1; M is a metal; Ln is a neutral oranionic ligand; and j is the number of ligands bound to M, wherein whenj is greater than 1, the Ln may be the same or different.
 26. The methodof claim 23, wherein the carbene precursor has the structure of formula(PIII)

wherein: E¹, E², E⁴, and E⁵ are independently selected from N, NR^(E),O, P, PR^(E), and S, R^(E) is hydrogen, heteroalkyl, or heteroaryl, x,y, v, and w are independently zero, 1, or 2, and are selected tocorrespond to the valence state of E¹, E², E⁴, and E⁵, respectively, andwherein when E¹ and E⁴ are other than O or S, then E¹ and E⁴ may belinked through a linking moiety to form a heterocyclic ring, and when E²and E⁵ are other than O or S, then E¹ and E⁵ may be linked through alinking moiety to form a heterocyclic ring; R¹, R², R⁸, and R⁹ areindependently selected from branched C₃-C₃₀ hydrocarbyl, substitutedbranched C₃-C₃₀ hydrocarbyl, heteroatom-containing branched C₄-C₃₀hydrocarbyl, substituted heteroatom-containing branched C₄-C₃₀hydrocarbyl, cyclic C₅-C₃₀ hydrocarbyl, substituted cyclic C₅-C₃₀hydrocarbyl, heteroatom-containing cyclic C₁-C₃₀ hydrocarbyl, andsubstituted heteroatom-containing cyclic C₁-C₃₀ hydrocarbyl; L¹, L², L⁴,and L⁵ are linkers containing 1 to 6 spacer atoms, and are independentlyselected from heteroatoms, substituted heteroatoms, hydrocarbylene,substituted hydrocarbylene, heteroatom-containing hydrocarbylene, andsubstituted heteroatom-containing hydrocarbylene; and h, k, m, and n areindependently zero or
 1. 27. The method of claim 1, wherein the polymeris a polycarbonate.
 28. The method of claim 1, wherein the polymer is apolyurethane.